The present invention relates to a method and system for performing handover in a cellular mobile communication system.
In a mobile telephone system like the GSM network, a telephone call is usually transmitted to a mobile station (MS) via a fixed network (Public Switched Telephone Network, PSTN), a mobile switching center, one of a plurality of radio network controllers (RNC, in GSM known as base station controllers BSC) and one of a plurality of base stations (BS). Therein, every individual base station serves a predetermined cell area.
In case a mobile station moves across a cell border between adjacent base stations, the current set of services associated with the mobile station have to be transferred from the current serving base station to a new serving base station. Such an operation is called xe2x80x9chandoverxe2x80x9d.
FIG. 1 schematically shows a known handover processing in a mobile communication system comprising a mobile station MS, two adjacent base stations BS1 and BS2, a switching means 4 for switching a telephone call from a connected network element to one of the two adjacent base stations, and a switching control means (RNC) 5 for controlling the switching operation. In the context of this application, the concept xe2x80x9cnetwork elementxe2x80x9d refers to any element of the mobile telecommunication system excluding the mobile station MS.
In the above known system, handover processing information is transmitted from the currently serving base station BS1 to the adjacent base station BS2 via the switching means 4 (cf. broken arrows in FIG. 1), when the mobile station MS moves across a corresponding cell border. To achieve this, a broadcast control channel (BCCH) is provided, by means of which the mobile station MS may detect the BS with the strongest signal.
In urban environments, the field strength of radio reception varies to a large extent due to multipath propagation and the lack of a line of sight. Thus, small cell sizes, i.e. micro/pico-cells, have to be provided so as to assure proper. reception of radio signals.
However, a connection-with the current serving base station BS1 may get lost very quickly in such a micro/picocell environment. This is very likely in the case of fast-moving mobile stations like cars which often suddenly change their moving directions at crossings, such that bearers of a mobile station cannot be properly handed over to the new base station BS2.
In this respect, the term bearer refers to a service realized on a radio interface. A radio bearer can be seen as a realization of the service, i.e. from layer 3 in the fixed network to the corresponding protocol in, the MS. Thus, layer 3 provides radio bearers as services: t higher layers of the connection layer model.
It is therefore an object of the present invention to provide a method for performing handover which ensures proper execution in microcell environments.
This object is achieved by a method for performing handover, comprising the steps of:
determining at least one candidate base station to which a mobile station is likely to be handed over; and
setting up an additional connection from a network element to said at least one candidate base station before executing handover.
Additionally, this object is achieved by a system for performing handover in a mobile communication system, comprising:
control means for determining at least one candidate base station to which a mobile station is likely to be handed over; and
switching means for setting up an additional connection from a network element to said at least one candidate base station before executing handover.
Accordingly, by determining candidate base stations and setting up additional connections from a network element to the candidate base stations in advance, a point-to-multipoint downlink data flow can be established to both the current serving base station and the determined candidate stations before the handover is executed. This pre-initialized handover ensures that handover processing data is available to all candidate stations, so that a proper handover is possible even in case of sudden interruptions of the connection with the current serving base station.
The determination step may be performed by means of a handover prediction on the basis of a moving direction and/or speed of the mobile station.
Base stations which are in a line-of-sight of the current serving base station do not have to be determined as candidate base stations, since a pre-reserved handover is not required.
Preferably, a list for identifying said at least one candidate base station can be transmitted to a switching control means in order to establish the required additional connections.
Moreover, mobile station connection parameters required for the connection with the mobile station can be transmitted to the switching control means, wherein a point-to-multipoint connection is set up to the current serving base station and the corresponding candidate base station(s). The mobile station connection parameters required for the possible handover can be transmitted to the candidate base station(s).
Furthermore, downlink user data transmitted through said point-to-multipoint connection could be buffered in the candidate base station(s), such that data loss during handover can be prevented.
Additionally, the candidate list can be updated by a current serving base station by checking whether said prediction is still valid. The connection parameters of valid candidate base station are transmitted to the mobile station.
In case of any change of connection parameters such as a bearer quality of service (QoS), connection parameter change information can be transmitted to said candidate base station(s) so as to ensure a proper handover execution.
In addition, a connection parameter version number can be transmitted to a candidate base station to which handover execution is to be performed and a connection parameter update is performed in dependence on the transmitted version number. Thereby, the connection parameters of the candidate base station can be adapted to those of the mobile station upon handover execution.
Upon handover execution, an established connection between a previous serving base station and the mobile station can be deleted, when a predetermined period expires without any attempt of the mobile station to contact the previous serving base station.